As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries as an energy source for the mobile devices has also sharply increased. Based on external and internal structures thereof, the secondary batteries are generally classified into a cylindrical battery, a prismatic battery, and a pouch-shaped battery. As the mobile devices have been miniaturized, the prismatic battery and the pouch-shaped battery, which have a small width to length ratio, have attracted considerable attention recently.
The prismatic secondary battery is manufactured by placing an electrode assembly including cathodes, anodes, and separators in a prismatic battery case, fixing a base plate to the upper end of the battery case, for example, by welding, injecting electrolyte into the battery case through an electrolyte injection hole formed in the base plate, sealing the electrolyte injection hole with a metal ball, mounting a safety element and a protection circuit on the base plate, and sealing the battery case with a housing (an external case).
A plan view of a base plate mounted to the upper end of a battery case of a conventional prismatic secondary battery with a vertical sectional view taken along line A-A is shown in FIG. 1.
Referring to FIG. 1, a base plate 100 is provided at the middle thereof with an electrode terminal 101, which is connected to an electrode tab (for example, an anode tab) of an electrode assembly, such that the electrode terminal 101 protrudes from the base plate 100. The base plate 100 is provided at one side thereof with an electrolyte injection hole 102, through which an electrolyte is injected. Between the protruding electrode terminal 101 and the base plate 100 is disposed an insulating member 103 to electrically isolate the electrode terminal 101 from the base plate 200, which is connected to the other electrode tab (for example, a cathode tab) of the electrode assembly so as to serve as an electrode terminal.
The electrolyte injection hole 102, through which the electrolyte is injected, is formed generally in a quadrangular shape in vertical section as shown in FIG. 2. The electrolyte injection hole 102 is sealed with a sealing member 104, made of aluminum, having a diameter slightly greater than that of the electrolyte injection hole 102. Specifically, the sealing member 104 is located on the electrolyte injection hole 102, and then the sealing member 104 is pressed from above such that the sealing member 104 is plastically deformed to seal the electrolyte injection hole 102. Subsequently, the periphery of the plastically deformed sealing member 104 is laser welded, or a thin metal plate is placed over the plastically deformed sealing member 104 and is laser welded, to seal the electrolyte injection hole.
When the sealing member is plastically deformed and is inserted into the electrolyte injection hole, which is formed in a quadrangular shape in vertical section, to seal the electrolyte injection hole with the sealing member, however, opposite sides of the sealing member are pushed outward by the inside upper end of the electrolyte injection hole. As a result, a groove is formed on the sealing member. The groove negatively affects the sealing of the electrolyte injection hole by laser welding. Furthermore, inside cracks are continuously formed along the interface between the sealing member and the inside surface of the electrolyte injection hole due to formation of the groove with the result that sealability of the electrolyte injection hole is lowered.
Also, in a case in which the electrolyte injection hole is sealed using a metal ball as the sealing member as described above, an electrolyte may leak out of the battery cell.
In addition, welding is further carried out to provide higher sealing force with the result that a defect rate of batteries is increased and manufacturing costs of the batteries are also increased.
Consequently, there is a high necessity for a technology that is capable of fundamentally solving the above-mentioned problems.